Filter elements produced with filter mat end seals are used for filtration of liquids, such as hydraulic fluid or solutions of all kinds such as suspensions, dispersions, emulsions, or colloidal solutions. They have generally proved to be successful because of their compact structure and their large filter area. To make certain that the fluid flow is conducted only through the filter medium itself, in addition to the seal on the two axial ends, the connection interface of the two opposite ends of the filter mat, in the form of end webs, form one of the critical points which are to be closed and made leakproof.
Known filter elements readily obtained through commercial channels may have a filter mat made up of various filter materials. The filter materials are folded in a zigzag fashion or are pleated, and are then mounted around a solid support tube provided with openings and enclosed in a fine-meshed wire screen following the fold contour of the filter mat as protection from external damage. In a part of the known processes for production of the known filter elements, the connection interface of the filter mat ends, as well as the connection of the filter mat to the ends, are obtained by a bed of an adhesive, such as one in the form of an epoxy resin adhesive. Waiting periods are needed until the adhesive becomes capable of bearing a load, depending on its gelling or cure time. The filter can only be used after its setting period as determined by the adhesive employed.
EP-B-0 001 407 discloses a process for production of a tubular corrugated filter element provided with at least one sheet of filter cloth. The filter cloth edges forming a side connection are stacked and are connected to each other to form a side edge seal by a self-supporting groove-like strip. The strip, as an adhesively connecting strip, comprises a thermoplastic resin which encloses the edges of the filter sheet. The strip is adhesively joined to the filter sheet. The edges of the filter sheet are sealed together in an interlocking seal so as to be leakproof, as a result of which escape of an unfiltered fluid between the edges of the of filter sheet of the tubular filter element is prevented. The connecting strip forms a U-shaped longitudinal clip which is welded to the ends of the filter mat by heating plates after being subjected to heat and pressure.
Consequently, the adhesive connections described for the preceding disclosed process, as well as the accompanying production costs are avoided. The free limbs of the strip-like connecting clip are designed to be flexible. These limbs may be moved apart when the ends of the filter mat are inserted so that the clamping pressure required may be applied to them for fastening. At the point of passage of the two free limbs of the longitudinal seam clip, the clip forms a stop against which the free ends of the filter mat ends come into contact and/or by which their movement in the direction of insertion into the clip is limited. The filter mat ends are flush with each other and assume the specified position required for production of the welded connection in the form of a smooth seam. However, despite the steps provided for this purpose, the filter mat ends may be forced together during introduction of the configuration into a welding device, with the result that the quality of the weld seam to be applied may be appreciably lowered. Consequently, the filter element may subsequently fail in the area of the welded seam points when in use. This is true in particular of filter elements subjected to high stresses when the weld seam connection is separated, something which may result both in failure of the filter element and in failure of an entire hydraulic assembly in which that filter element has been installed.
DE-A-196 51 242 discloses the anvil of an ultrasonic sealing unit for sealing at least two fields of material formed of backing material coated with plastic. A sonotrode is movable relative to the anvil. The seal surface on the front of the body of the anvil is contoured by raised and lowered areas. With this solution, it is not possible to seal or weld tubular filter elements and their filter materials by means of ultrasound. Also, a smooth weld seam is applied on the free end of the surface materials to be connected. This welding is favorable for milk cartons, since such cartons are designed to be opened easily by hand in the area of the smooth weld seam. Reliability in operation, even under high load, cannot be achieved with the solution disclosed.
WO 01/05483 A1 (corresponding to pending U.S. patent application Ser. No. 09/979,865) discloses a generic process for production of a connection interface in a tubular filter element having a pleated filter mat. The opposite filter mat ends are fixed in position by a connecting strip which overlaps the ends at least in part. The connecting device comprises a thermoplastic which is welded to the ends of the filter mat. The welding process is carried out without a welding filler, and connects the clip to the ends of the filter mat in the fused state. In the welding process, an impression is made in the connecting strip, in the form of the plastic clip, at least along one of its two sides such that the filter mat ends are welded to each other in compressed form. Since the welding process can be carried out in an extremely short time, simultaneously with introduction of the embossing, efficient production without use of additional epoxy resin adhesives or the like is possible. Since no holding or gelling times arise, the element may be used immediately or may immediately undergo additional processing.
Another result of the embossing is that the filter mat ends are welded together in compressed form so that an especially strong and lasting connection is obtained for the filter mat ends, even if these ends are subjected to very high fluid pressures during filtration. Since the filter mat ends are pulled apart beyond the embossed welding seam in the direction of the pleated elements of the filter mat because of the internal stress of the material, a wide gap occurs in this area. The wide gap impairs the sealing efficiency, and may be a factor if the embossed weld seam as produced is not entirely clean and is forced apart so that passages may be formed for undesirable escape of fluid. Introduction of material stresses into the embossed weld seam through the additionally widening gap occurs in the transitional area between pleating and embossed weld seam, so that the respective weld seam undergoes introduction of high punctate or linear forces to such an extent that failure may also occur, particularly if the filter element is subjected to alternating pressure stresses during operation.